Does it?
I think Markdown "won" for a variety of reasons (Github? Slack? Pandoc?). I don't think AsciiDoc is so much more complicated than Markdown that people can't learn it quickly.
The reason Markdown is now standard is that people like standardized things. They want to use the same syntax in their chatroom, forum, ad-hoc readme filed, and doc comments in code. And if they're already using one markup language, why learn another? Why not use the same one everywhere?
It's not that AsciiDoc is complicated in general, it's that it's ever so slightly too rigid for "ad-hoc" markup in places like Discord and Hacker News. Thus people end up learning Markdown because it really is the best fit for those platforms, and then people don't want to learn another markup language, and so you get a snowball effect of implementation support and popularity. And thus we are stuck with Markdown everywhere.
This isn't necessarily a bad outcome. But that's how I see it. I still want AsciiDoc to gain more traction in technical writing!